The reversion frequencies of most genetically-altered bacterial pathogens preclude their use as live vaccines. We propose to solve this problem by introducing multiple mutations of identical phenotype into a single strain. The reversion frequency of such a strain will be the product of the reversion rates of the individual mutations. Hence, reversion rates of 10 to the minus 21th power can be achieved, making the strain safe. We have demonstrated the feasibility of this approach by constructing a strain of Haemophilus influenzae with three temperature-sensitive mutations of identical phenotype (Hooke, A.M. thesis, Georgetown University, 1979). We are using this technology to construct a set of attenuated strains of H. influenzae (HI) for immunogenic evaluation and ultimate production of a liver vaccine. Attenuation of type b HI is achieved by incorporation of three temperature-sensitive (ts) mutations of identical phenotype into each candidate strain. In all a set of nine strains will be constructed to evaluate the effects of cut-off temperature and limited residual growth at the restrictive temperature on vaccine efficacy.